The storage portfolio in California is intended in part to support the transition to a low-carbon electric power sector. As such, it is expected that over the long-term, storage operations would reduce carbon emissions by enabling more energy from renewable resources. This section examines this question for the LTPP scenarios by first examining changes in all resource
operations by fuel type across the WECC due to storage operations, then discussing the effect on aggregate generator emissions within California, and finally the aggregate emissions for the WECC region.
Changes in Generation by Fuel Type
One of the significant sources of value is energy storage charging with lower fuel cost plants and displacing higher fuel cost plants. In many regional simulations of hourly storage operations, the ability of storage to charge from lower fuel cost and high-emissions generation, such as coal plants, can lead to higher total emissions [2, 14]. As described in Section 4, given the regional scope of the LTPP model, along with the attempt to reflect existing carbon policies, modeling the impact of storage on regional carbon emissions is complicated. Because there is no carbon tax outside California, the model tends to shift charging energy to the lowest cost plants, which are often coal-fired plants outside California, despite the inclusion of a carbon tax in the wheeling charge into California.28 Figure 16 and Figure 17 show the changes in generation from
28 In the LTPP model, the wheeling charge for carbon emissions from imports into California is represented as a
constant value and is independent of the generation mixture in the exporting region. As a result the wheeling charge does not send a clear incentive to import generation from a lower carbon source. See carbon cost in Section 4.1 for more details on the wheeling charge formulation.
generators across WECC for the 33% and 40% scenarios. The addition of storage reduces curtailment, which increases generation from renewables. However, as discussed previously, there is very little curtailment in the 33% scenario and so little opportunity for increased renewable generation. This opportunity increases for the 40% scenario, and presumably would increase further at even higher RPS. Additionally, due to storage round-trip efficiency energy losses, there is an overall increase in generation for all cases except reserves only (which is due to that case not accounting for energy required to provide reserves). Much of the change in emissions is associated with how the added storage causes the thermal fleet to re-dispatch in order to minimize production cost.
As shown in the figure, the impact on different types of generation is related to the storage applications. When storage provides reserves, it allows for the greatest reductions in gas-fired generation from CCs but also the largest increases in out-of-state coal generation. Since the majority of operating reserves are provided by in-state CCs operating at part-load, storage providing reserves allows the system to reduce generation from CCs and increase lower cost imports, which include increases in coal generation. Conversely, if storage can only provide energy then it cannot displace in-state CCs, which are needed to provide reserves. More details about reserve provision can be found in Appendix C. In all cases, storage displaces generation from CTs as storage displaces peaking generation. There is also a change in existing storage generation for both the reserves scenarios and the energy and reserves scenarios. At 83.3% efficiency, the proposed storage portfolio has higher efficiency and greater flexibility to provide reserves than existing storage. As a result, installing the proposed storage portfolio displaces a portion of the existing storage generation.
Figure 17: Change in yearly generation mixture for 40% scenarios with storage for all of WECC
California Generation Emissions
The model allows us to isolate emissions from in-state California generation. Generator
emissions are calculated from normal operation as well as startup, based on individual plant fuel type and heat rate. Figure 18 shows the change in emissions within California due to storage operations. Annual California generator emissions are reduced in every case, and reductions range from 325,000 tons carbon dioxide (CO2) to as high as 1.1 million tons CO2 in the 40%
case. The largest emissions reduction is in the reserves only scenario, while providing arbitrage services only achieves around 40% of the maximum emissions reduction.
Figure 18: Change in emissions for the core 33% and 40% scenarios with storage for California only
WECC Emissions
When the changes in aggregate WECC emissions are evaluated for each of the use-cases, the results are mixed but also fairly small. In some cases, we find regional emissions increases due to increases in coal generation outside California discussed previously. Figure 19 shows these aggregate emissions, which are the sum of the changes in emissions for each resource type shown in Figure 16 and Figure 17. The overall annual impact on emissions is very small, ranging from a decrease of 275,000 tons to an increase of 223,000 tons.29 This can be compared to the
total emissions for the WECC of 310 and 301 million tons of CO2, and 45 and 44 million tons
for California (33% and 40%, respectively). It is important to note that these impacts do not consider the make-up energy requirements for storage providing regulation or load following (which would reduce the benefit of storage) or the avoided generator cycling (for example, when fast-responding storage provides frequency regulation which would increase the benefit of storage). These issues are discussed in more detail in Section 6.1 and [13].
29 For the purposes of comparison, if the additional 1,325 MW of storage were to charge only with renewables, and
displace the natural gas combined cycle (with a discharge capacity factor of 15%), the avoided emissions would equal about 1.7 million tons of CO2. The lower values estimated here are largely due to the limited amount of
Figure 19: Change in emissions for the core 33% and 40% scenarios with storage for the Western Interconnect
In sum, the results for these scenarios suggest that storage operations will decrease in-state emissions, and also regional emissions for use-cases which shift more energy as renewable energy increases in the 40% scenario. There are a number of other factors that could affect the impact of storage on regional emissions in the timeframe of 2024, including Clean Power Plan regulations and other policies which could change the western resource mix and regional market integration. Regardless, the impact of the storage mandate on regional emissions will likely be relatively small over the coming decade when compared to other policies, such as increasing RPS. In addition, the ability of storage to reduce emissions will depend on both increased availability of low-carbon electricity for charging, as well as policies or other resource changes that reduce the opportunities for charging from the remaining coal generation in the West. Overall, these results suggest that as renewable penetration increases both in California and throughout the West, regional carbon emissions can be reduced by storage operations.